The present application is based on Japanese priority application No.2001-128373 filed on Apr. 25, 2001, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to a semiconductor device having a ferroelectric capacitor and the process of fabricating the same.
A ferroelectric memory device (FeRAM) is a non-volatile semiconductor memory device that can hold information even when the electric power is turned off. In an FeRAM, writing of information is achieved by utilizing the hysteresis of ferroelectrics.
A typical FeRAM has a ferroelectric capacitor in which a ferroelectric film is sandwiched by a pair of electrodes, and information is written into the ferroelectric film by causing a polarization therein in response to application of an electric voltage across the electrodes. The information thus written into the ferroelectric film in the form of spontaneous polarization is retained even when the application of the electric voltage is terminated.
In such a ferroelectric capacitor, the polarity of the spontaneous polarization is reversed when the polarity of the applied voltage is inverted. Thus, by detecting the direction of the spontaneous polarization, it becomes possible to read out the written information.
An FeRAM can be operated with a lower driving voltage as compared with the case of a flash memory, but with much higher speed and with much lower electrical power consumption.
In the fabrication process of an FeRAM, it is necessary to conduct thermal annealing processes in an oxygen atmosphere repeatedly for recovering degradation of characteristics of the ferroelectric film, which occurs as a result of processing conducted in a non-oxidizing atmosphere. It should be noted that the ferroelectric film constituting the ferroelectric capacitor easily undergoes formation of oxygen defects when exposed to a non-oxidizing atmosphere. Associated with this, the characteristics of the ferroelectric film such as switching electric charge, leakage current, the like, are degraded altogether.
In view of the use of the oxygen atmosphere, conventional FeRAMs have used a metal not susceptible to oxidation in an oxygen atmosphere, such as Pt, IrOx, RuOx, or other conductive oxides, for the upper electrode.
Meanwhile, FeRAMs are subjected to severe demand of device miniaturization similarly to other semiconductor devices. Associated with this, there are demands for miniaturization of the ferroelectric capacitor. Further, there are demands for using a multilayer interconnection structure also in FeRAMs. Furthermore, there are demands of low voltage operation for FeRAMs, particularly in relation to application for mobile information processing.
In order to achieve a low-voltage operation for an FeRAM, it is necessary that the ferroelectric film constituting the ferroelectric capacitor has a large switching electric charge QSW. In the case that a multilayer interconnection structure is used, however, there arises a problem in that the characteristic of the ferroelectric capacitor undergoes serious degradation as a result of the processes used for forming the multilayer interconnection structure. It should be noted that these processes for forming a multilayer interconnection structure have to be conducted under a reducing atmosphere or under a non-oxidizing atmosphere.
More specifically, in the case the upper electrode is formed of a Pt film or an Ir film, or the like, hydrogen contained the reducing atmosphere invades into the Pt film or the Ir film and undergoes activation by the catalytic action of these metals at the time of forming an interlayer insulation in the multilayer interconnection structure. Thereby, there arises a problem in that the ferroelectric film in the ferroelectric capacitor is reduced by the hydrogen radicals that have been thus activated. When the ferroelectric film undergoes such a reduction, the operational characteristics of the ferroelectric capacitor are degraded seriously as noted before. This problem of degradation of the characteristics of the ferroelectric film appears particularly conspicuously when the ferroelectric capacitor is miniaturized and the capacitor insulation film in the ferroelectric capacitor is formed of a miniaturized ferroelectric pattern.
Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor device having a ferroelectric capacitor and the process of making the same, wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a fabrication process of a semiconductor device having a ferroelectric capacitor enabling miniaturization the ferroelectric capacitor while suppressing the degradation of electric characteristics of a ferroelectric film in the ferroelectric capacitor and simultaneously enabling formation of a multilayer interconnection structure on the ferroelectric capacitor.
Another object of the present invention is to provide a semiconductor device, comprising:
a substrate; and
a ferroelectric capacitor provided over said substrate,
said ferroelectric capacitor comprising a lower electrode, a ferroelectric film provided over said lower electrode, and an upper electrode provided over said ferroelectric film,
said upper electrode comprising: a first layer of an oxide having a stoichiometric composition represented by a formula AOx1 by using a compositional parameter x1 and an actual composition represented by a formula AOx2 by using a compositional parameter x2; and a second layer provided on said first layer and having a stoichiometric composition represented by a formula BOy1 by using a compositional parameter y1 and an actual composition represented by a formula BOy2 by using a compositional parameter y2,
wherein there holds a relationship
y2/y1 greater than x2/x1. 
Another object of the present invention is to provide a fabrication process of a semiconductor device, comprising the steps of:
forming a lower electrode;
depositing a ferroelectric film on said lower electrode;
depositing a first conductive oxide film on said ferroelectric film; and
depositing a second conductive oxide film on said first conductive oxide film,
wherein said step of depositing said first conductive oxide film is conducted under a less oxidizing condition as compared with said step of depositing said second conductive oxide film.
According to the present invention, there occurs a diffusion of Pb into the lower layer part of the upper electrode from the ferroelectric film as a result of the use of the first conductive oxide film having a non-stoichiometric composition for the lower layer part of the upper electrode. It should be noted that the lower layer part of the upper electrode makes a contact with the ferroelectric film forming the capacitor insulation film of the ferroelectric capacitor. As a result, the interface between the ferroelectric film and the lower layer part of the upper electrode is effectively palanarized, and the effective voltage applied to the ferroelectric film is increased when a voltage is applied across the ferroelectric capacitor. On the other hand, such a conductive oxide film having a non-stoichiometric composition causes a problem, when exposed to an atmosphere containing hydrogen, in that the metallic components contained in the film cause activation of hydrogen in the atmosphere, and the hydrogen thus activated induces degradation of characteristics of the ferroelectric film. Because of this, the present invention forms the upper layer part of the upper electrode layer located on the lower layer part of the upper electrode layer, by using the second conductive oxide film having a stoichiometric composition or a composition closer to the stoichiometric composition than the first conductive oxide film. In this way, the penetration of the reducing atmosphere to the lower layer part of the upper electrode is effectively blocked.
According to the present invention, miniaturization of a ferroelectric capacitor can be realized in an FeRAM having a multilayer interconnection structure while suppressing the degradation of the ferroelectric film.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.